[Tuor of Gondolin]

"One who cannot cast away a treasure at need is in fetters."
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There are obviously strong arguments for trying to hold Minas Tirith, both literary and strategic. The concept of a retreat and strategic withdrawal is more in the nature of "contingency plans" and leaving your options open. It would obviously be better to hold Minas Tirith but to stubbornly do so at all costs would be analagous to Stalin's insisting on holding forward front lines and then Kiev in 1941, rather then the flexible defense favored by his better generals. And recall that at Stalingrad the majority of the civilian polulation was withdrawn. Certainly Gondor did not anticipate the gate being destroyed, and if it hadn't a viable defense was possible (though not certain).
Denethor did (unlike PJ's characterization) make prudent preperations. One can imagine in the weeks and months preceding a possible attack evacuations of nonessential populations. And of intriguing interest to me is the use Gondor and Rohan could have made of pretty secure back links to each other and as a fairly safe area (even if in camps) for their populations in the Anfalas and Pinnath Gelin.
If the corsairs were repulsed the nature of the rivers in south Gondor and strongholds like Dol Amroth and Pelargir would have been substantial barriers to a divided Mordor force and given time for an even further withdrawal of refugees into Eriador, if necessary. And as it happened, with Aragorn destroying the corsairs and occupying south Gondor past Pelargir (and controlling the seas) a strong defensive stance. A costly, but possibly necessary, strategy, if the alternative was to stay in an untenable forward area subject to conquest, perhaps again like the Russian withdrawal of population, industry, etc. east. A very costly strategy, but what was the alternative, to be destroyed by a still superior wehrmacht?
Remember. in Rhovannion men and dwarves withdrew into Erebor, abndoning Laketown and thereby survived.